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Rhododendrons (Rhododendron spp.) are stunningly beautiful but temperamental shrubs that are notably susceptible to pests and diseases if not planted in ideal growing conditions. The sheer number of species, hybrids and cultivars on the market, topping 3,000, is a testament to the fact that many gardeners consider them well worth the effort.

Rot

Root and crown rot are serious fungal diseases that occur in overly wet conditions or compacted, poorly draining soil. Symptoms include general plant wilting, curled, drooping leaves and rotten black roots. According to the American Rhododendron Society, once infected, some varieties never fully recover despite the gardener's best efforts. Fungicides containing metalaxyl can help control the disease. Consider planting rot resistant cultivars in the future such as "Hampton Beauty," suitable for U.S. Department of Agriculture plant hardiness zones 7 to 10, or "Glacier," hardy in USDA zones 6B to 9.

Foliage Diseases

Foliage diseases such as rust and leaf spot lower the ornamental value of the shrub, though they usually cause no serious problems. Remove and destroy dropped leaves to help prevent the disease from spreading. Applying a preventive fungicide containing copper can be helpful for leaf spot. Poor air circulation can contribute to foliage problems, so remove weeds from around the plant. Leaves may curl if temperatures drop below 35 degrees F, but will generally recover after a cold snap.

Pests

Weevils and mites are among the most common rhododendron pests, sometimes causing foliage drop. Weevils chew on the edges of leaves, causing distinct looking notches, while mites chew on inner sections, causing scores of white dots. Weevils may be plucked off by hand in the evening, when they are most active, or a flypaper-like barrier can be stuck on the main trunk. Mites may be controlled with regular applications of horticultural oil. According to the American Rhododendron Society, hybrid rhododendrons are more susceptible to weevils.

Prevention

Rhododendrons must be planted in the proper cultural conditions in order to thrive. Diseases and pests are often indicators of a poor environment. Hardiness varies widely between varieties, generally falling somewhere between U.S. Department of Agriculture zones 4 to 9, so make sure you plant the right rhododendron for your climate. Rhododendrons require a well-draining, acidic soil. A generous layer of pine needles or leaves will help protect the shrub's shallow roots. Partial shade is suitable for most species.

About the Author

Michelle Wishhart is a writer based in Portland, Ore. She has been writing professionally since 2005, starting with her position as a staff arts writer for City on a Hill Press, an alternative weekly newspaper in Santa Cruz, Calif. An avid gardener, Wishhart worked as a Wholesale Nursery Grower at Encinal Nursery for two years. Wishhart holds a Bachelor of Arts in fine arts and English literature from the University of California, Santa Cruz.